The present invention relates to a method for connecting multiple battery cells of a battery. The invention also relates to a battery system having a battery having multiple battery cells, each battery cell having a respective associated battery cell monitoring module arranged in the battery.
FIG. 1 depicts a battery system 10 that is known from the prior art and that comprises a battery 11 having multiple battery cell units (Smart Cell Unit SCU) 20 that each have a battery cell 21 and a battery cell monitoring module (battery cell electronics module or battery cell electronics) 22 associated with the battery cell 21. To simplify the depiction from FIG. 1, only two battery cell units are outlined and each provided with the reference symbol 20. The battery cell monitoring modules 22 allow individual control of the single battery cells 21. To produce an output voltage (total output voltage) U from the battery 11, which is also used as the output voltage U of the battery system 10, the battery cell monitoring modules 22 are connected to one another in a series circuit by means of a link. The battery system 10 further comprises a central control unit (CCU) 30 for controlling the battery system 10.
To produce a regulated output voltage (total output voltage) U from the battery 11, single battery cells 21 are each switched on by means of the associated battery cell monitoring module 22, that is to say that the battery cells 21 are each introduced into the series circuit or electrically coupled to the battery 11 in positive or negative polarity relative to the tap of the output voltage U. To produce a regulated output voltage (total output voltage) U from the battery 11, single battery cells 21 are further each switched off by means of the associated battery cell monitoring module 22, that is to say that the battery cells 21 to be switched off are isolated from the series circuit or are electrically decoupled from the battery 11 by virtue of the connection terminals of each battery cell 21 to be switched off being electrically connected by means of the associated battery cell monitoring module 22, as a result of which the applicable battery cells 21 are bypassed. The battery cells 21 connected to form the series circuit may consequently each be in a switching state referred to as “positively connected” or in a further switching state referred to as “negatively connected”. Further, the battery cells 21 isolated from the series circuit are in a switching state referred to as “bypassed”.
In such battery systems 10 (SmartCell battery systems), the decision about the change of switching state for the battery cells 21 is made locally in the respective battery cell monitoring modules 22. The actual regulatory function is implemented by the central control unit 30, which is in the form of a central controller, implemented with little complexity.
In this case, a first controlled variable P1 and a second controlled variable P2 are prescribed in the battery system 10 by means of a communication link 31, in the form of a unidirectional communication interface, that the central control unit 30 uses to send only a single message, which comprises the present controlled variables P1 and P2, to all battery cell monitoring modules 22. All battery cell monitoring modules 22 receive the same message and either autonomously connect the respective associated battery cells 21 of the series circuit or bypass the respective associated battery cells 21 using the applicable switches that are present in each of the battery cell monitoring modules 22 (not depicted). According to a control algorithm, the central control unit 30 prescribes the two controlled variables P1, P2 in the form of two numerical values between 0 and 1 that are transmitted via the communication link 31 from the central control unit (CCU) 30 to the battery cell monitoring modules (SCU) 22 and are equally received by all battery cell monitoring modules 22. In this case, it holds that 0≦P1≦1 and 0≦P2≦1.
FIG. 2 shows a control engineering equivalent circuit diagram 15 for the battery system 10 that depicts production of a desired or prescribed output voltage Us of the battery 11 of the battery system 10.
FIG. 2 reveals that the central control unit 30 provides the controlled variables P1, P2 for each battery cell monitoring module 22 of the battery 11. In this case, the value of the first controlled variable P1 and the value of the second controlled variable P2 are regularly prescribed and updated by the central control unit 30 at a constant update rate or update frequency. Consequently, the first controlled variable P1 and the second controlled variable P2 are transmitted at a transmission rate or transmission frequency that is the same as the update frequency.
In this case, whenever the value of the first controlled variable P1 and the value of the second controlled variable P2 are updated, an applicable evenly distributed random process is carried out in each battery cell monitoring module 22, which random process interprets the respective present value of the first controlled variable P1 as the present value of a first probability, referred to as the switch-on probability, of the associated battery cell, when switched off, being switched on and that interprets the respective present value of the second controlled variable P2 as the present value of a second probability, referred to as the switch-off probability, of the associated battery cell 21, when switched on, being switched off. Consequently, this associated battery cell, after the applicable random process has been carried out, will remain either switched off or switched on in each case, that is to say will maintain its switching state, or be switched on or switched off, that is to say will change its switching state. The value of the currently produced output voltage U appears in each case on the basis of the switching states that the battery cells 21 of the battery 11 have after the applicable random processes are carried out. In FIG. 2, Zi denotes the switching state that the ith battery cell 21 has after an applicable random process is carried out. Further, U denotes the output voltage currently produced by the battery 11 and Us denotes the desired output voltage that is currently to be produced. In addition, ΔU denotes the existing control difference or control error between the currently produced output voltage U and the desired output voltage Us that is currently to be produced from the battery 11. In this case, the central control unit 30 tracks the controlled variables P1 and P2 such that the magnitude of said control difference or control error ΔU is as small as possible.
FIG. 3 shows an example of a profile of an output voltage U produced, measured in volts, as a function of a time t, measured in milliseconds, in comparison with a further profile of a desired output voltage Us, to be produced in the form of a sinusoidal AC voltage and also specified in volts, as a function of the time t. In this case, regulation of the output voltage U produced has been performed here in the same way as the regulation described in connection with FIG. 2.
Since the update frequency at which the value of the first controlled variable P1 and the value of the second controlled variable P2 are updated is the same as the transmission frequency at which the first controlled variable P1 and the second controlled variable P2 are transmitted, said update frequency is limited by the maximum transmission frequency of the transmission channel, which in this case is in the form of the communication link 31. Said transmission frequency should, however, be chosen to be as high as possible in order to obtain a low magnitude for the control error ΔU and a small time constant for regulation as cited above. This is of great significance particularly for producing a desired output voltage Us to be produced in the form of an AC voltage. To produce a sinusoidal output voltage U from the battery 11, for example, the controlled variables P1, P2 should be sent to the battery cell monitoring modules 22 repeatedly per half-cycle of the sinusoidal desired output voltage Us to be produced as applicable. To obtain as few harmonics as possible in the sinusoidal output voltage (sinusoidal oscillation) U produced from the battery 11, the update frequency at which the value of the first controlled variable P1 and the value of the second controlled variable P2 are updated is chosen to be higher than the frequency of the sinusoidal desired output voltage Us to be produced as applicable by at least a factor of 10.
A disadvantage of using a high update frequency for the value of the first controlled variable P1 and for the value of the second controlled variable P2 is that the switching state of each switched-off battery cell 21 of the battery 11 or of each switched-on battery cell 21 of the battery 11 can possibly change on receipt of each new value of the first controlled variable P1 or of the second controlled variable P2 by the respective battery cell monitoring module 22 associated therewith. The switching processes connected to such switching state changes or switching state variations cause switching losses in the power electronics that each of the applicable battery cell units 20 contain, which switching losses reduce the amount of energy that can be taken from such a battery system 10 and a corresponding battery system efficiency. In addition, these switching processes increase the requirements on cooling of the power electronics or increase the additional heating of each battery cell 21 by the applicable power electronics.
The document US 2012/0038322 A1 discloses a battery having a battery management system and having multiple battery modules, each battery module comprising multiple battery cells, an analog-to-digital converter and a switching apparatus. In this case, the battery management system is designed to set a switching rate at which the switching apparatus of each battery module specifically prompts each battery cell of the applicable battery module to communicate with the analog-to-digital converter present in the applicable battery module on the basis of at least one parameter of the applicable battery cell.